There is an increasing demand for character recognition from an image taken by a camera or the like. As a method to recognize characters contained in an image, a technique with high recognition accuracy called Sliding Window is known, for example (see Non Patent Literature 1, for example). This technique sets a window with a certain size for character recognition and performs character recognition in the set window, and it scans the window all over the detection target image repeatedly while changing the window size little by little.